To consolidate the ground, for example by underpinning, or for sealing the ground, for example underneath dams or excavation pits, use is made of injection technology which is also known as jet grouting. Jet grouting is a soil grouting method wherein an energy-rich cutting jet consisting of water and a cement suspension is used to cut and erode the soil contained in the region of the borehole. The cutting jet which can also be surrounded by air is injected into the soil at high exit speeds. The eroded soil is repositioned and mixed with cement suspension. Part of the mixture is flushed through the annular space of the borehole towards the mouth of the borehole. This method can be used to produce construction elements of very different geometric shapes. Depending on the type of soil, the process applied and the liquid used, the width of erosion of the jet in the ground can be anything up to 2.5 meters.
The jet grouting process is used in special foundation engineering. Because of the nature of the process, the process of producing consolidated ground columns does not allow any visual monitoring. As a result, any non-uniformities in the soil structure or changes in the process parameters in the course of production can lead to a lesser quality of the ground column produced. This is the reason why, as a rule, trial columns are produced and analyzed prior to the start of the actual jet grouting work. There are different prior art methods of determining the diameter of a ground column produced by jet grouting, for instance thermal diameter measurements or hydrophone recordings.
From DE 195 21 639 A1 a method of monitoring a high-pressure injection process is known. A high-pressure injection rod with an exit nozzle for a high-pressure jet is driven into the soil. By lifting and simultaneously rotating the rod, the soil is cut open by the high-pressure jet and mixed with injection material. During the jetting operation, the soil vibrations in the vicinity of the high-pressure injection rod are recorded by a hydrophone. For this purpose, a gauge is driven into the soil in the surroundings of the high-pressure injection rod at a distance which corresponds to the expected range of the high-pressure jet. Said gauge is a pipe which is closed at its lower end, which is filled with water forming an incompressible medium and in which the hydrophone is guided. During the jetting operation, the hydrophone is guided at the level of the exit nozzle so that it is reached by the high-pressure jet exiting radially relative to the high-pressure injection rod. The resulting soil vibrations are converted to an analogue signal. In this way, it is possible to obtain information on the range of the high-pressure jet, which information is used to specifically vary the parameters of the jetting process.
From AT 505 438 A1 a method of determining the radial expansion and the content of hydraulically bonding materials of members produced by jet grouting (DSV members) is known. For this purpose, a first temperature curve is measured in a predeterminable period of time in a first range of the DSV-body; said temperature curve is then compared with reference curves. These measured temperature curves then allow to draw conclusions regarding the measurements and strength of the DSV column produced.
Thermal temperature determining methods require long preparatory periods of time, and there exists a further disadvantage in that an immediate evaluation cannot take place. The method of determining a diameter by means of a geophone can only be used down to a certain depth which, depending on the type of soil, amounts to approximately 8 to 15 meters. Furthermore, the rod and microphone have to be lifted synchronously, which requires additional technical facilities and which also leads to measuring inaccuracies if the two pieces of equipment are lifted at different speeds.